1. Field of the Invention
The present invention relates to a memory device, and more particularly, to a resistive memory cell, a method of forming the same, and a resistive memory array using the same.
2. Description of the Related Art
Generally, a process of forming a semiconductor includes a thin film deposition process, a diffusion process, an oxidation process, an ion implantation process, a photolithographic process, an etching process, and so forth. Among these processes, the photolithographic process includes forming a photoresist pattern on a substrate. The photoresist pattern corresponds to a circuit pattern that has been designed in advance. An exposed underlying thin film is etched using the photoresist pattern formed on the substrate as an etch mask so that the circuit pattern is transferred to the thin film of the substrate.
The photolithographic process for forming the photoresist pattern on the substrate includes a coating process of a photoresist material, an exposure process, and a development process. The photoresist material exposed to light is photochemically changed, and thus, an exposed portion and a non-exposed portion of the photoresist material have structures chemically different from each other. Therefore, by using an appropriate developing solution, one of the two portions is selectively removed, and the other portion, which is not removed by the developing solution, becomes a photoresist pattern.
The photoresist pattern formed by the exposure and development process is removed by an ashing process, a stripping process, or the like, after it has been used as an etch mask for etching underlying layers. The ashing is a process of removing the photoresist pattern using oxygen plasma in a plasma etching apparatus, and the stripping is a process of removing the photoresist pattern at about 125° C. using a mixed solution of sulfuric acid and an oxidant. In removing the photoresist pattern, the photoresist pattern should be removed as fast as possible, without effecting the underlying pattern.
A problem with respect to the process of removing the photoresist pattern will be set forth with regard to the fabrication of a memory cell.
Recently, attempts have been made to fabricate a resistive memory cell by employing an organic material as a memory element. FIG. 1 schematically illustrates a cross-point memory array employing an organic material as the memory element. Referring to FIG. 1, in the organic memory cell array, a plurality of parallel upper electrodes 15 intersect a plurality of parallel lower electrodes 11. Between the upper electrodes 15 and the lower electrodes 11, an organic thin film 13, which acts as an memory element, is disposed. Therefore, a unit memory cell is configured with the upper electrode 15, the lower electrode 11, and the organic material 13 disposed at an intersection region of the upper and the lower electrodes 15 and 11. A method of forming the cross-point memory array of FIG. 1 includes: forming the lower electrode 11; forming the organic thin film 13; depositing a conductive layer for the upper electrode; and patterning the conductive layer to thereby form the upper electrode 15. The patterning of the conductive layer for the upper electrode 15 requires a photolithographic process and a photoresist pattern removal process such as an ashing or a stripping process.
To highly integrate the resistive memory cells using the organic material, it is very important for the memory element such as the organic material to have thermal, mechanical and chemical stability without deterioration, even after performing the photolithographic process and the photoresist pattern removal process as state above.
However, the organic material used as the memory element is apt to deteriorate during the photolithographic process and the photoresist removal process, because the ashing process is a high temperature oxidation process using energy of plasma particles and reactive radicals, and the stripping process is performed using a chemical solution. The deterioration of the organic material due to the photoresist removal process causes the operational characteristics of the resistive memory cell to deteriorate also. For example, such deteriorated operational characteristics include ambiguity to distinguish between two resistance states, or an increased leakage current, or that a high voltage is required for a switching operation, or the like.
Moreover, if the organic thin film used as the memory element is not separated into every memory cell, then a leakage current may occur between neighboring memory cells. This may be a limiting factor in increasing the degree of integration.